1. Field of the Invention
The Invention relates to the precision measurement of circular objects, particularly to fasteners having external or internal threads, such as the cylindrical or tapered threads on a bolt or a nut. The Invention also applies the measurement of a cylindrical or tapered object, such as a rod, or a circular space, such as a bored hole. The Invention includes a method of measuring an object.
The precision measuring apparatus and method of the Invention is most applicable to those industries requiring precision measurement of fasteners, such as the aerospace industry and the medical devices industry.
2. Background of the Invention
In this application, a fastener such as a bolt with external threads to be measured is referred to as an ‘external thread.’ A fastener such as a nut with internal threads to be measured is referred to as an ‘internal thread.’ A ‘circular object’ includes any object with a circular cross section, such as a cylindrical or tapered object or external thread, and any object having a circular opening, such as a bored hole or an internal thread.
Prior art precision thread gauges incorporate two stationary gauge rolls and one movable gauge roll. See, for example, U.S. Pat. No. 6,381,861 B1 to Deterling, issued May 7, 2002, and U.S. Pat. No. 4,974,327 to Greenslade, issued Dec. 4, 1990. For purposes of this application, the term ‘gauge roll’ generally means the portion of the gauge that physically touches the object to me measured. The gauge roll may be a designated portion of a component, such as a table as hereinafter defined. Alternatively, the gauge roll may be a separate part of the apparatus that is releasably attached to another component of the apparatus. Where the object to be measured is an external thread, the gauge roll is a piece of hard material, such as carbide or hardened tool steel, formed to match the profile of an external thread to be measured. For example, if the external thread to be measured has ten turns per inch, the gauge rolls used to measure that external thread also will exhibit a profile of ten turns per inch.
Gauge rolls for measuring threads are conventional in the art and different types of gauge rolls may be selected to measure different aspects of a thread. Gauge rolls also may be polygonal (for example, square) in cross section, may be formed as a vertically or horizontally disposed blade, may be tapered (for measuring a tapered thread or other tapered circular object) and may take the form of a sharp point. As used in this application, ‘gauge roll’ includes all configurations for a gauge roll. For simplicity, the gauge rolls illustrated in this application are ‘full form functional’ gauge rolls and are cylindrical in form and generally circular in cross section.
In the prior art thread gauge, the single movable gauge roll is advanced toward the two stationary gauge rolls until all three gauge rolls mesh with and engage the external threads. The position of the movable gauge roll with respect to the fixed gauge rolls is noted.
The prior art thread gauge must be set up to measure a particular diameter of external thread (hereinafter the “set up diameter”) by selecting gauge rolls of a precise size. If the diameter of a measured external thread is precisely the size of the set up diameter, then the prior art gauge works well. If the external thread to be measured is smaller or larger than the precise set up diameter, then it is difficult to determine the actual size of the external thread from the measurement provided by the prior art gauge.
The shortcoming of the prior art gauge lies in its geometry. One way to visualize the problem with the prior art gauge is to consider that each of the three gauge rolls has at lease one point of contact with the circular object being measured. Corresponding points of contact on the three gauge rolls together define a triangle. Because two gauge rolls are fixed and the third gauge roll moves with respect to the other two, the interior angles of that triangle change as the movable gauge roll is moved. Because of the changing angles, determining the actual diameter of an object that is smaller or larger than the set up diameter becomes a complex exercise in trigonometry.